The present invention relates to communications systems applying slow frequency hopping, and more particularly to a procedure for synchronizing two frequency hopping units to each other in order to establish a communication link.
Frequency hop (FH) spreading has been an attractive communications form in military applications for a long time. By sending signals sequentially in different parts of the radio spectrum in a pseudo-random way, both high security against eavesdropping, and immunity against narrowband interferers is obtained. With the advent of fast, cheap, and low-power synthesizers, FH transceivers are becoming commercially attractive, and are used more and more in civil applications as well. For certain wireless radio systems, FH is especially attractive because of its immunity to unknown interference and to Rayleigh fading. Examples are radio systems using unlicenced bands like the Industrial, Scientific and Medical (ISM) bands at 900, 2400 and 5700 MHZ. Because the radio communications are unregulated in these bands (apart from some transmission power restrictions), communication systems using this band must be capable of sustaining any (i.e., a priori unknown) interference. FH appears to be an attractive tool in fighting the interference.
Two types of FH systems can be distinguished: slow FH and fast FH. In slow-FH communications, a burst of symbols is transmitted in a hop. So the symbol rate is higher than the hop rate. In fast-FH, a single symbol is spread over several hops, so that the hop rate is higher than the symbol rate. Fast-FH puts high requirements on the speed of the transceiver electronics, especially at higher symbol rates. Therefore fast-FH is not attractive for portable usage because of higher power consumption. Slow-FH provides all the system features required in a wireless communications system, that is, interference immunity and fading immunity.
For a FH connection to operate, synchronization between the two hopping transceivers is required: the transmission (TX) hop of one unit must be the receive (RX) hop of the other unit, and vice versa. Once the two units are locked, they just use the same hop sequence at the proper rate in order to maintain the connection. However, a problem is to get the two units synchronized initially. When there is no connection, a portable unit is usually in a standby mode. In this mode, it sleeps most of the time, but periodically it wakes up to listen for paging messages from units that want to connect. A problem with a FH scheme is that the paging unit does not know when and on what hop channel the unit in standby will listen for paging messages. This results in an uncertainty both in time and in frequency.
Conventional techniques have attempted to solve the problem of establishing a connection between a paging unit and a unit in standby mode. In U.S. Pat. No. 5,353,341 issued to Gillis, a single reserved hop channel is used for access. The paging unit always sends paging messages out on this single reserved channel, and when the standby unit periodically wakes up, it only monitors the one reserved channel. Because there is no hopping of the access channel, there is no frequency uncertainty. However, this strategy has the drawback of lacking the benefits that an FH strategy can provide: When the reserved channel is disturbed by a jammer, no access can take place.
U.S. Pat. No. 5,430,775 to Fulghum et al. discloses a system in which reserved channels are used as agreed upon by sender and recipient. In this case, there are two reserved channels: one to xe2x80x9creservexe2x80x9d the access channel, and the other is the access channel itself. The access process lacks the benefits that FH can provide because both the reservation and the access channel do not hop, but are instead constant.
U.S. Pat. No. 5,528,623 to Foster, Jr. discloses a system in which both the sender and the recipient hop in the access procedure, thereby providing the fall benefits of a FH scheme. However, in this system the recipient is required to hop quickly during the wake-up period, while the paging unit hops slowly. As a result, this system has the undesirable effect of requiring the recipient (i.e., the unit in standby) to expend a relatively large amount of power during every wake-up period, just to check to see whether it is being paged. Another apparent shortcoming of the system as described by Foster, Jr. is that there is no explanation of how the return message from the recipient to the sender is arranged. That is, a 3.3 ms return period is defined in which the sender listens for a response; but upon receipt of the page message, the recipient does not know when this 3.3 ms listening period starts.
It is therefore an object of the present invention to provide an access method for units applying a FH scheme which allows the standby unit to have a low duty cycle on the sleep/wake-up period thus providing a low-power standby mode, but at the same time limits the access delay in setting up a connection.
The foregoing and other objects are achieved in apparatuses and methods for establishing a connection between a paging unit and a standby unit in a channel hopping communications system. In accordance with one aspect of the invention, the standby unit is activated for an activation time period, Twake, out of every standby time period, Tstandby. During each activation time period, the standby unit is caused to monitor a selected channel for receipt of a paging message, wherein the selected channel is selected from a plurality of channels, and wherein, for each subsequent activation time period, the selected channel is a subsequent one of the plurality of channels as specified by a hopping sequence. During a first repetition period, a first page train is repeatedly transmitted from the paging unit to the standby unit, until a response is received from the standby unit. If the response is not received from the standby unit during the first repetition period, then during each of one or more subsequent repetition periods, a corresponding one of one or more subsequent page trains is repeatedly transmitted from the paging unit to the standby unit until the response is received from the standby unit. In the above technique, each of the first and subsequent page trains comprises a plurality of paging messages, each paging message being transmitted on a different one of a subset of the plurality of channels. The first page train is transmitted on a subset of channels that are selected from the hopping sequence, wherein the selected channels include a hop frequency associated with an expected wake-frequency and one or more different hop frequencies that are nearest the expected wake-frequency in the hopping sequence, and wherein non-selected channels in the hopping sequence constitute one or more remaining portions of the hopping sequence. Each of the one or more subsequent page trains is transmitted on a respectively different subset of channels that are selected from those channels that are nearest the expected wake-frequency in successively remaining portions of the hopping sequence.
In one aspect of the invention, the repetition period may be substantially equal to the standby period. In alternative embodiments, the repetition period may be greater than or equal to the standby period.
In another aspect of the invention, each page train is transmitted on a subset of channels that are selected from the plurality of channels in accordance with the following equation:
train i={hopmodN(ksxe2x80x2xe2x88x92(i+1)M/2), hopmodN(ksxe2x80x2xe2x88x92(i+1)M/2+1), . . . , hopmodN(ksxe2x80x2xe2x88x92iM/2xe2x88x921)
hopmodN(ksxe2x80x2+iM/2), hopmodN(ksxe2x80x2+iM/2+1), . . . , hopmodN(ksxe2x80x2+(i+1)M/2xe2x88x921)}
where
ksxe2x80x2 is an estimate of a clock value of the standby unit, the standby unit""s clock value being updated every Tstandby period,
N is the number of channels in the hopping sequence,
Tpage is the duration of a page message,
M=INT(Twake/Tpage)xe2x88x921, where INT( ) is a function that leaves only the integer part of a variable,
the number of page trains, NT, is given by NT=RNDUP(N/M), where RNDUP( ) is a function that rounds any non-integer up to the nearest integer,
i=0, . . . , (NTxe2x88x921),
and hopmodN(x)=hop(x mod N). This technique for selecting page train channels is especially useful when M is an even number.
In an alternative embodiment, each page train is transmitted on a subset of channels that are selected from the plurality of channels in accordance with the following equations:
train i={hopmodN(ksxe2x80x2xe2x88x92iM/2xe2x88x92(Mxe2x88x921)/2), . . . , hopmodN(ksxe2x80x2xe2x88x92iM/2xe2x88x921), hopmodN(ksxe2x80x2+iM/2), hopmodN(ksxe2x80x2+iM/2+1), . . . , hopmodN(ksxe2x80x2+iM/2+(Mxe2x88x921)/2)}
when i is an even number within the range 1 . . . EVEN(NTxe2x88x921); and
xe2x80x83train i={hopmodN(ksxe2x80x2xe2x88x92iM/2xe2x88x921/2xe2x88x92(Mxe2x88x921)/2), . . . , hopmodN(ksxe2x80x2xe2x88x92iM/2xe2x88x923/2), hopmodN(ksxe2x80x2xe2x88x92iM/2xe2x88x921/2), hopmodN(ksxe2x80x2+iM/2xe2x88x921/2+1), . . . , hopmodN(ksxe2x80x2+iM/2xe2x88x921/2+(Mxe2x88x921)/2)}
when i is an odd number within the range 1 . . . ODD (NTxe2x88x921),
wherein:
EVEN(x) represents a first function that returns x when x is even, and returns a value xxe2x88x921 when x is odd;
ODD(x) represents a second function that returns x when x is odd, and returns a value xxe2x88x921 when x is even;
ksxe2x80x2 is an estimate of a clock value of the standby unit, the standby unit""s clock value being updated every Tstandby period;
N is the number of channels in the hopping sequence;
Tpage is the duration of a page message;
M=INT(Twake/Tpage)xe2x88x921, where INT( ) is a function that leaves only the integer part of a variable;
the number of page trains, NT, is given by NT=RNDUP(N/M), where RNDUP( ) is a function that rounds any non-integer up to the nearest integer;
and hopmodN(x)=hop(x mod N). This technique for selecting page train channels is especially useful when M is an odd number.
For any of the above embodiments, and in another aspect of the invention, the estimate of the standby unit""s clock may be determined from a present clock value of a paging unit clock, adjusted by a previously determined offset between standby unit and paging unit clock values.
In yet another aspect of the invention, for any of the above embodiments, the previously determined offset may be stored in a non-volatile memory for future access attempts.
In still another aspect of the invention, the channel hopping communications system may be a frequency hopping communications system. In alternative embodiments, the channel hopping communications system may be a code hopping communications system.